Description of the Prior Art
Among the various types of fuel cell systems are those which include subassemblies of two bipolar plates between which is supported an electrolyte, such as an acid, in a matrix. The subassemblies, herein referred to as fuel cells, are oriented one atop another and electrically connected in series to form a fuel cell stack. Operation of the fuel cell, for example the reaction of hydrogen and oxygen to produce electrical energy as well as water and heat, is exothermic, and cooling of the cell components is necessary in order to maintain component integrity. For example, the bipolar plates or the electrolyte matrix may be made of carbonaceous material bonded by a resin which tends to degrade at high temperatures. Prolonged operation at high temperatures would tend to degrade many components of a typical fuel cell. Further, the exothermic reaction can result in uneven temperature distribution across a fuel cell, thus limiting cell-operating temperature and efficiency, and additionally raising concerns about catalyst poisoning, for example, by carbon monoxide.
Accordingly, fuel cell systems have in the past been proposed with closed liquid cooling loops. Typically proposed are systems comprising a plurality of stacked cells where every fourth cell or so includes small metallic tubing through which cooling water is recirculated. Circulatory power is accordingly required, detracting from overall cell efficiency. This is complicated by large pressure drops in small diameter tubing, and the susceptibility of the cooling tubes to attack by mediums within the cell stack, such as acids in certain designs.
Also proposed are systems wherein large amounts of an oxidant, such as air, in quantities which are multiples of the stoichiometric amount necessary to carry out the electrochemical reaction, are circulated through a stack of fuel cells to additionally function as a cooling medium. As with liquid-cooled systems, an associated penalty is the large amount of circulatory power required.
More recently proposed have been systems including a stack of fuel cells with a cooling module placed between every fourth or so fuel cell in the stack. Air is manifolded so as to flow in parallel through the process oxidant channels of the fuel cells, as well as through cooling passages of the cooling module. The cooling module passages are much larger than the fuel cell process channels so that approximately eighty percent of the air flows through the cooling cell passages and the balance through the process cell channels. While such systems represent an improvement in terms of mechanical power requirements, additional improvements can be made. For example, where the amount of airflow is reasonable, that is, where an amount which does not require excess circulatory power is utilized, the air flowing through the cooling channels absorbs substantial amounts of heat energy as the cooling passage is traversed, resulting in less cooling at the exit end of the channel. This condition results in an uneven temperature profile in the fuel cell stack and attendant unbalanced reaction rates, voltage and current distributions, and limits maximum operating temperatures.
It is therefore desirable to provide improved cooling arrangements for stacked fuel cell systems which preferably do not suffer excessively high pressure drops and circulatory power requirements and which provide for better temperature distribution throughout the fuel cell stack.